Transmission system for OFDM-signals with optimized synchronization

ABSTRACT

The present invention relates to a transmission apparatus for transmitting OFDM-signals comprising modulation means  4  for modulating said signals onto a plurality of subcarriers using a OFDM-modulation method, transformation means  5  for transforming said modulated signals into the time domain, and transmission means for transmitting said signals, whereby in said modulation means every M-th subcarrier is modulated, wherein M is an integer and M≧2. The present invention also relates to a corresponding transmission method for transmitting OFDM-signals. 
     The present invention further relates to a receiving apparatus for receiving OFDM-signals comprising M identical or respectively mirrored wave forms within one OFDM-timeburst, wherein M is an integer and M≧2, comprising receiving means for receiving said OFDM-signals, correlation means  22  for correlating said wave forms to obtain time synchronization, transformation means  23  for transforming said signals into the frequency domain and demodulation means  24  for demodulating said signals. The present invention also relates to a corresponding receiving method for receiving OFDM-signals. The present invention provides a much better time and frequency synchronisation performance based on correlation techniques than conventional OFDM-systems.

More than one reissue application has been filed for the reissue of U.S.Pat. No. 6,731,594. The reissue application are application Ser. Nos.11/416,477 (Parent reissue application), 12/621,543 (present divisionalreissue application) and 12/621,544 (second divisional reissueapplication)

The present invention relates to a transmission method according to thepreamble of claim 1, to a transmission apparatus according to thepreamble of claim 5, a receiving method according to claim 9, areceiving apparatus according to claim 13 and a transmission systemaccording to claim 17.

In a conventional OFDM-system signals or information contained insignals are modulated onto subcarriers in the frequency domain. Thespacing between the subcarriers is equal and the subcarriers arearranged orthogonally in the frequency domain. The respectively appliedmodulation scheme varies for example the magnitude and phase of thedescribed subcarriers. A conventional transmission apparatus fortransmitting OFDM-signals therefore comprises as basic elementsmodulation means for modulating said signal onto a plurality ofsubcarriers using a OFDM-modulation method, transformation means fortransforming said modulated signals into the time domain, andtransmission means for transmitting said signals. In a conventionalOFDM-system, a transmission means for OFDM-signals extends a time domainsignal after a transformation into the time domain (e. g. by an inversediscrete Fourier transformation) by some guard samples to overcomemultipath effects during the transmission. Usually the extension of thetime domain signal is done by a cyclic extension, wherein a part of thewave form is repeated. A corresponding OFDM-signal receiving apparatuscan perform correlation utilizing the two identical wave form parts toobtain information on the timing of the OFDM-time bursts for furtherprocessing. Usually this timing information is used to optimally placethe discrete Fourier transformation window in the receiving apparatus tobe able to transform the modulated subcarriers into the frequency domainand to demodulate them thereafter.

To provide an efficient transmission system, the guard time or cyclicextension has to be as short as possible, namely slightly larger thanthe longest expected transmission path duration, which can result inpoor cyclic extension based correlation properties in a receivingapparatus if the cyclic extension is very short (e. g. only a fewsamples). In this case, in known OFDM-systems synchronization bursts areused, which contain only synchronization information. This reduces thetransmission efficiency, since a special synchronization burst designedin the time domain does not contain information (in thefrequency/subcarrier domain) to be transmitted.

The object of the present invention is therefore to provide atransmission method according to the preamble of claim 1, a transmissionapparatus according to the preamble of claim 5, a receiving methodaccording to claim 9, and a receiving apparatus according to claim 13,which provide optimized correlation possibilities.

This object is achieved by a transmission method according to claim 1, atransmission apparatus according to claim 5, a receiving methodaccording to claim 9, and a receiving apparatus according to claim 13.Also, this object is achieved by a transmission system according toclaim 17.

The transmission method for transmitting OFDM-signals according to thepresent invention comprises the steps of modulating said signals onto aplurality of subcarriers using a OFDM-modulation method, transformingsaid modulated signals into the time domain, and transmitting saidsignals, characterized in that in said modulating step every M-thsubcarrier is modulated with a signal, wherein M is an integer and M≧2.

The transmission apparatus for transmitting OFDM-signals according tothe present invention comprises modulation means for modulating saidsignals onto a plurality of subcarriers using a OFDM-modulation method,transformation means for transforming said modulated signal into thetime domain, and transmission means for transmitting said signals,characterized in that in said modulation means every M-th subcarrier ismodulated, wherein M is an integer and M≧2.

The receiving method according to the present invention is adapted forreceiving OFDM-signals comprising M identical or respectively mirroredwave forms within one OFDM-timeburst, wherein M is an integer and M≧2,and comprises the steps of receiving said OFDM-signals, correlating saidwave forms to obtain time synchronization, transforming said signalsinto the frequency domain, and demodulating said signals.

The receiving apparatus according to the present invention is adaptedfor receiving OFDM-signal comprising M identical or respectivelymirrored wave forms within one OFDM-timeburst, wherein M is an integerand M≧2, and comprises receiving means for receiving said OFDM-signals,correlation means for correlating said wave forms to obtain timesynchronization, transformation means for transforming said signals intothe frequency domain, and demodulation means for demodulating saidsignals.

Advantageous features of the present invention are defined in therespective subclaims.

The modulation of every M-th subcarrier according to the presentinvention, after the succeeding transformation of the signals into thetime domain, e. g. by an inverse discrete Fourier transformation,results in a signal containing M identical or respectively mirrored waveforms, whereby the total duration of the OFDM-timeburst is still 1/f₀(f₀ is the subcarrier spacing). With M identical wave forms within oneOFDM-timeburst, the corresponding receiving apparatus can perform anoptimized correlation in the time domain, e. g. to obtain time andfrequency information and synchronization, respectively. Further on,information to be transmitted can be modulated onto every M-thsubcarrier and the transmission of a special time-domain synchronizationtime burst usually not containing useful information in thefrequency-subcarrier domain is not necessary.

The present invention can be applied to every transmission system basedon a multicarrier OFDM-modulation method, e. g. wireless and wiredtransmission systems. Possible and advantageous applications of thepresent invention in a wireless transmission system are for example theRACH (Random Access Channel), the BCCH (Broadcast Control Channel), andthe IACH (Initial Acquisition Channel). Generally, the present inventionis especially effective in scenarios where conventional algorithms toimprove correlation based time synchronization, e. g. averaging overmultiple time bursts is not possible. The present invention can beapplied to any OFDM-system, particularly, when a robust timesynchronization for further signal processing, e. g. discrete Fouriertransformation, is required.

Advantageously, in said modulation means the not modulated subcarriersare set to zero. Further advantageously, only subcarriers with evenindices are modulated. If only subcarriers with even indices aremodulated (e. g. M=2), a full (complex) time domain signal consisting oftwo equal wave forms is obtained after the transformation into the timedomain (e. g. by an inverse discrete Fourier transformation). If, on theother hand, only subcarriers with odd indices are modulated (e. g. M=2),a full (complex) time domain signal after the transformation into thetime domain is obtained, which contains two respectively mirrored waveforms. In this case, the two wave forms are mirrored so that thecorrelation result is negative and an additional absolute value unit (orinverter) is necessary in the receiving apparatus to achieve a positivecorrelation result and a correct frequency offset.

Advantageously, said modulation means comprises means for generatinginteger values from 0 to L−1, wherein L is the number of availablesubcarriers, whereby said modulation means modulates every M-th signalonto said subcarriers on the basis of said integer values.

Advantageously, in the correlation means of the receiving apparatusaccording to the present invention, the identical or respectivelymirrored wave forms are correlated on the basis of a delay value L1=S/Mand averaged over L2≦S/M samples, whereby S is the total number ofsamples in one OFDM-timeburst.

It is further advantageous in the receiving apparatus according to thepresent invention to provide a peak detection means after saidcorrelation means for providing time synchronization for thetransformation of said signals into the frequency domain. It is furtheradvantageous to provide a frequency offset detection means after saidcorrelation means for providing frequency synchronization for thetransformation of the signals into the frequency domain.

The transmission system for transmitting OFDM-signals according to thepresent invention comprises a transmission apparatus according to thepresent invention and a receiving apparatus according to the presentinvention. This transmission system can be based on a wireless or wiredtransmission of signals.

The present invention is explained in detail by means of preferredembodiments relating to the enclosed drawings, in which

FIG. 1 shows an-embodiment of the transmission apparatus according tothe present invention,

FIG. 2 shows the modulation unit of the transmission apparatus shown inFIG. 1 in more detail,

FIG. 3 shows an example of modulating every 4-th subcarrier with asignal in the frequency domain,

FIG. 4 shows an example for a signal comprising four identical waveforms in the time domain,

FIG. 5 shows an embodiment of a receiving apparatus according to thepresent invention,

FIG. 6 shows the time/frequency synchronization means of the receivingapparatus shown in FIG. 5 in more detail and in a general form,

FIG. 7 shows the time/frequency synchronization means of the receivingapparatus shown in FIG. 5 for a signal comprising two identical waveforms,

FIG. 8 shows the time/frequency synchronization means of the receivingapparatus shown in FIG. 5 for a signal comprising four identical waveforms,

FIG. 9 shows the frequency offset detection means in more detail,

FIG. 10 shows a frequency spectrum for a conventional correlationperformed on the basis of a cyclic extension of a time burst, and

FIG. 11 shows a frequency spectrum for a correlation according to thepresent invention for a random access channel.

FIG. 1 shows an embodiment of a transmission apparatus according to thepresent invention. In the transmission apparatus shown in FIG. 1, data 1are channel coded in a channel coding means 2 and interleaved in aninterleaving means 3. In a modulation unit 4, the signals carrying thedata to be transmitted are modulated with an OFDM-modulation method. AnOFDM-system is a multicarrier system with a plurality of subcarriers. Inthe modulation unit 4, the signals carrying the information to betransmitted are modulated on every M-th subcarrier, wherein M is aninteger and M≧2. The modulated signals, for example APM-signals,amplitude-phase-modulated signals, are transformed into the time domainin an inverse discrete Fourier transformation means 5. After thetransformation into the time domain, the transformed signals areprovided with a cyclic extension in a cyclic extension means 6a and thenshaped in a burst shaping means 6b. In the cyclic extension means 6a,the OFDM-time bursts are provided with a guard time (=cyclic extensionof the signal) to mitigate multipath effects during transmission. Thiscyclic extension serves also to provide correlation (to achieve time andfrequency synchronisation) in a corresponding receiving apparatus. Thecyclic extension consists in a part of the signal being added to the endof the signal, so that the receiving apparatus can conduct calculationson the basis of the doubled signal parts to provide correlation. Theburst shaping means 6b does not have to be provided in the transmissionapparatus according to the present invention, since the describedcorrelation method (to achieve time and frequency synchronisation) isbased on the cyclic extension only. The provision of the burst shapingmeans 6b, however, improves the transmission spectrum (reduced out ofband spurious emission).

After the burst shaping means 6b, or, if the burst shaping means 6b isnot provided, after the cyclic extension means 6a, the signals aredigital/analog-converted in a D/A-converter 7 and then RF-upconverted ina RF-upconversion means 8 to be transmitted by an antenna 9.

In FIG. 2, the modulation means 4 of the transmission apparatus shown inFIG. 1 is shown in more detail. The transmission means 4 comprises asubcarrier number generator 10 for generating integer values 0,1 . . .L−1 corresponding to the available subcarrier number L in one frequencyslot in the OFDM-system. The integer values generated by the subcarriernumber generator 10 are fed to a modulation unit 17. Also, the integervalues generated by the subcarrier number generator 10 are fed to amodulo means 11, which generates series of integer values depending onthe chosen modulation step of the modulation means 4. If, for example,every 4-th subcarrier is modulated with a signal, so that M=4, themodulo means 11 outputs integer values 0,1,2,3,0,1,2,3,0,1,2,3, . . . .

The output of the modulo means 11 is fed to a compare means 12, whichcompares the integer values provided by the modulo means 11 with integervalues generated by a compare value generator 13. The compare means 12gives an “active” signal to a switch means 14, if the inputs from themodulo means 11 and the compare value generator 13 are equal. If, forexample in the above example, the compare value generator 13 generatesan integer value “1”, the compare means 12 outputs an “active” signalevery 4-th time an integer value “1” is fed from the modulo means 11(M=4). Otherwise, the output of the compare means 12 is a “not active”signal. If the switch means 14 obtains an “active” signal from thecompare means 12, it connects a line 16 providing signals with data tobe modulated with the modulation unit 17. If the switch means 14 obtainsan “not active” signal from the compare means 12, it connects a zeroterminal 15 with the modulation unit 17. In the above example (M=4), theswitch means 14 therefore connects the data line 16 every 4-th time aninteger value is generated by the subcarrier number generator 10 withthe modulation unit 17. Therefore, every 4-th subcarrier is modulatedwith signals carrying data in the modulation unit 17. The othersubcarriers are not modulated in the modulation unit 17, since theswitch means 14 selects the zero terminal 15 at the time thesesubcarriers are fed to the modulation unit 17. At the zero terminal 15,a “0” value is input (complex: 0=0+j×0) so that the other subcarriersare not modulated.

In FIG. 3, a frequency domain representation for the modulation of every4-th subcarrier is shown. The horizontal axis shows the number S=32 ofthe inverse discrete Fourier transformation samples 0 . . . 31 and thevertical axis shows the magnitude of the subcarriers. Also, onefrequency slot comprising L=24 (0 . . . 23) available subcarriers isshown wherein each subcarrier is sampled in the inverse discrete Fouriertransformation means 5. Each 4-th subcarrier 18 (subcarrier number 0, 4,8, 12, 16 and 20) is modulated with a signal, wherein the spacingbetween adjacent subcarriers is f₀. The IDFT samples 0 . . . 3 and 28 .. . 31 are unmodulated guard subcarriers (to perform a power-of-2 DFT,here 32-point DFT), and the samples 4 . . . 27 are the used subcarriersamples (here we assumed one frequency slot consists of 24 subcarriers).

FIG. 4 shows the corresponding time domain wave forms for the exampleshown in FIG. 3, wherein every 4-th subcarrier is modulated. Themodulation of every 4-th subcarrier leads to time domain signalscontaining 4 identical wave forms, since only subcarriers with evenindices (compare FIG. 3) have been modulated.

In FIG. 4A, the IN-part (in-phase part), and in FIG. 4B, the QUAD-part(quadrature part) of a wave form signal in the time domain, in whichevery 4-th subcarrier has been modulated in the frequency domain, isshown. FIG. 4C shows the envelope of the IN-part and the QUAD-part shownin FIG. 4A and FIG. 4B, respectively (envelope=SQRT {IN*IN+QUAD*QUAD}).As can be seen, the wave form signals contain 4 identical wave forms,since in the frequency domain only subcarriers with even indices havebeen modulated. The modulation of subcarriers with only odd indicesleads to wave forms which are slightly different to the wave forms shownin FIG. 4. The modulation of subcarriers with only odd indices leadsafter the transformation in the time domain to wave form signals withrespectively mirrored wave forms. In this case, every second wave formin the time domain signal is mirrored in respect to the correspondinglypreceding wave form. If a sample in a first waveform is x₁=a+j×b, thecorresponding sample in the second waveform is x₂=(−a−j×b)=(−1)*(a+j×b).

In FIG. 5, an embodiment of a receiving apparatus according to thepresent invention is shown. Data transmitted, e. g. from a transmissionapparatus as shown in FIG. 1, are received in an antenna 19 andRF-downconverted in a RF-downconversion means 20. Then, the signals areanalog to digital converted in an A/D-converter 21 and fed to atime/frequency synchronization means 22. In the time/frequencysynchronization means 22, the received signals are correlated andsynchronized, so that a proper transformation to the frequency domain ina succeeding discrete Fourier transformation means 23 can be executed.The transformed signals are then demodulated in a demodulation means 24.The demodulated signals are de-interleaved in de-interleaving means 25and then channel-decoded in a channel-decoding means 26. Thechannel-decoding means 26 outputs data signals 27 to be furtherprocessed.

In FIG. 6, the time/frequency synchronization means 22 of the receivingapparatus shown in FIG. 5 is shown having a general structure. Thetime-/frequency synchronization means 22 consists generally of acorrelation means with one or more correlator parts 28, 29, 30, 31 and amoving average means 40. After the moving average means 40, an absolutevalue means 45 is provided. After the absolute value means 45, a peakdetection means 46 can be provided. The output of the peak detectionmeans 46 and the output of the moving average means 40 can be fed to analso optionally provided frequency offset detection means 47.

The time/frequency synchronization means 22 comprises (M−1) correlatorparts. If, for example, every 4-th subcarrier is modulated, thetime-/frequency synchronization means 22 comprises 3 correlator parts,as is shown in more detail in FIG. 8.

In FIG. 6, the output of the A/D-converter 21 is fed to a firstcorrelator part 28 comprising a delay means 32 and a multiplier 35. Theoutput of the A/D-converter is fed to the delay means 32, which delaysthe signal with a factor z^(−L1). The output of the delay means and theoutput of the A/D-converter 21 are multiplied in the multiplier 35. Theoutput of the delay means 32 is further fed to a delay means 33 and amultiplier 36 of a second correlator part 29. The delay means 33 delaysthe output of the delay means 32 with a factor z^(−L1). The output ofthe delay means 33 is multiplied in the multiplier 36 with the output ofthe delay means 32. The outputs of the multiplier 35 and the multiplier36 are added in an adder 38. Successive correlator parts and adders aresymbolized by a block 30. The (M−1)th correlator part 31 delays theoutput of the delay means of the preceding correlator part in a delaymeans 34 by a factor z^(−L1) and multiplies the output of the delaymeans 34. The output of the delay means 34 is multiplied in a multiplier37 with the output of the preceding delay means. The output of themultiplier 37 is added in an adder 39 to the output of a precedingadder.

Then, the output of the last adder 39 is fed to the moving average means40. In the moving average means 49, the incoming signal is delayed in adelay means 41 by a factor z^(−L2). In an adder 42, the output of thedelay means 41 is subtracted from the incoming signal. The output of theadder 42 is fed to an adder 43, which is backfed with its own outputdelayed by factor z⁻¹ in a delay means 44. The moving average means thusperforms the function

${{\left( {1 - z^{{- L}\; 2}} \right)/\left( {1 - z^{- 1}} \right)} = {\sum\limits_{m = 0}^{L\; 2}\; z^{- m}}},$which means y(m)=x(m)+x(m−1)+. . . +x(m−L2) if the input signal of theMAV means 40 is defined as x(m) and its output signal is defined asy(m).

In the example of FIG. 6 and also the examples of FIGS. 7 and 8, thecorrelation delay value L1 is L1=S/M. The moving average value L2 isL2≦S/M, so that a signal fed to the moving average means 40 is delayedover L2≦S/M samples. In both cases, S is the total number of samples inone OFDM-timeburst. In the example shown in FIG. 3, S=32 and M=4, sothat L1=8 and L2≦8. The best performance is achieved if L2 is close toS/M, in the example of FIG. 3 this means L2 should be close to 8 samples(e. g. 6, 7 or 8 samples).

In the correlation means 28, 29, 30, 31 and the moving average means 40,correlation in the time domain to obtain time synchronizationinformation for further processing of the incoming signals is performed.The output of the moving average means 40 is then fed to an absolutevalue means 45. The output of the absolute value means 45 is fed to apeak detection means 46, which identifies the best correlation resultfor an optimum estimate of the window position of the discrete Fouriertransformation in the discrete Fourier transformation means 23. In anideal transmission case, the imaginary part of the correlated signal iszero. In the case of a frequency offset in the transmitted signal, theimaginary part of the correlated signal is not zero, so that a frequencyoffset detection has to be performed in a frequency offset detectionmeans 47. Conventionally, if all subcarriers are modulated, thefrequency offset detection range is limited to −f₀/2 . . . +f₀/2,whereby f₀ is the subcarrier spacing. According to the presentinvention, the frequency offset detection range in the frequency offsetdetection means 47 is extended to M×(−f₀/2) . . . M×(+f₀/2), wherein f₀is the subcarrier spacing. Therefore, the frequency offset detectionrange is advantageously extended according to the present invention. Theoutput of the frequency offset detection means 47 and the peak detectionmeans 46 are used for time-/frequency synchronization in the succeedingdiscrete Fourier transformation means 23.

In a case, in which only subcarriers with odd indices are modulated, anadditional absolute block means (or sign inverter) can be used in thereceiving apparatus to achieve a positive correlation result. Thisadditional absolute block means can, for example, be provided betweenthe last correlation part and the moving average means 40. In order toachieve time synchronisation only this block is not necessary, becausethe absolute value means 45 in FIG. 5 already provides positive results.However, to achieve a correct frequency detection (synchronisation),this additional absolute block means is required.

In FIG. 7, a time-/frequency synchronization means 22 is shown for M=2.In this case, the correlations means consists only of one correlatorpart 28. The correlation delay value L2 is S/2 and the moving averageparameter L2 is smaller or equal S/2, whereby the best performance isachieved if L2 is close to S/2.

In FIG. 8, a time-/frequency synchronization means 22 is shown for M=4.In this case, L1=S/4 and L2≦S/4.

In FIG. 9, the frequency offset detection means 47 shown in FIGS. 6, 7and 8 is shown in more detail. As stated above, the frequency offsetdetection range is advantageously extended according to the presentinvention. The structure of the frequency offset detection means 47shown in FIG. 9 provides this extended frequency offset detection range.

The frequency offset is Δf=M×f₀×(½π)×arctan(q/i), wherein M is thenumber of the repeated wave forms in one OFDM time burst, f₀ thesubcarrier spacing, “i” the in-phase part and “q” the quadrature part ofthe complex output of the MAV means 40. As shown in FIG. 9, thefrequency offset detection means 47 comprises a split means 48, acalculation means 49 and a multiplier 50. In the split means 48, thecomplex output of the MAV means 40 is separated in an “in” and a “quad”component, when the split means 48 receives a peak detection signal fromthe peak detection means 46. The peak detection means produces a peakdetection signal every time it detects a peak. The “in” and “quad”component from the split means 48 are then fed to the calculation means49. The calculation means 49 calculates the mathematical expression of(½π)×arctan(q/i), which can be done in a look-up table (hardwareimplementation) or calculated in a processor. The calculation resultfrom the calculation means 49 is supplied to the multiplier 50. Themultiplier 50 multiplies the calculation result from the calculatingmeans 49 with M(number of repeated wave forms in one OFDM time burst).The result of the multiplication in the multiplier 50 is the frequencyoffset Δf as a fraction of the subcarrier spacing f₀ (result=Δf/f₀). Thedetected frequency offset is used in the synchronisation unit 22 of thereceiving apparatus to obtain the frequency synchronisation.

In FIG. 10, a frequency spectrum of a conventionally correlated signal(cyclic extension) is shown and compared with a frequency spectrum shownin FIG. 11 for a signal correlated according to the present invention.The parameter for the example shown in FIG. 10 has been calculated for aRACH-burst. Its parameters are: signal to noise ratio: 6,0 dB, frequencyoffset: −0,30001×f₀, guard samples per burst: 16, RACH-scheme: 4, numberof RACH-slots: 4, discrete Fourier transformation size (=number ofsubcarriers or number of OFDM-burst samples): 128 and used subcarriersper slot: 96.

As can be seen, the present invention provides for very good peakdetection compared to the conventional correlation. The four bursts inthe signal stream can be clearly identified. The detected frequencyoffset values are: 0,3004; 0,3081, 0,3117 and 0,3151 which is veryaccurate (error<5%).

1. Transmission method for transmitting OFDM-signals, comprising thesteps of modulating said signals onto a plurality of subcarriers using aOFDM-modulation method, transforming said modulated signals into thetime domain, and transmitting said signals characterized in that in saidmodulating step every M-th subcarrier is modulated with a signal,wherein M is an integer and M≧2.
 2. Transmission method according toclaim 1, characterized in, that the not modulated subcarriers are set tozero.
 3. Transmission method according to claim 1, characterized in,that M=2 and only subcarriers with even indices are modulated. 4.Transmission method according to claim 1, characterized in, that saidmodulation step comprises the steps of generating integer values form 0to L−1, wherein L is the number of available subcarriers, and modulatingevery M-th signal onto said subcarriers on the basis of said integervalues.
 5. Transmission method according to claim 1, wherein: saidmodulating step includes providing a switch control signal to a switchhaving a first input and a second input, wherein the first inputreceives a signal to be modulated onto a subcarrier and the second inputreceives a zero value signal.
 6. Transmission apparatus for transmittingOFDM-signals, comprising: modulation means (4) for modulating saidsignals onto a plurality of subcarriers using a OFDM-modulation method,transformation means (5) for transforming said modulated signals intothe time domain, and transmission means for transmitting said signalscharacterized in that in said modulation means every M-th subcarrier ismodulated, wherein M is an integer and M≧2.
 7. Transmission apparatusaccording to claim 6, characterized in, that in said modulation means(4) the not modulated subcarriers are set to zero.
 8. Transmissionapparatus according to claim 6, characterized in, that in saidmodulation means (4) M=2 and only subcarriers with even indices aremodulated.
 9. Transmission apparatus according to claim 6, characterizedin that said modulation means (4) comprises means (10) for generatinginteger values from 0 to L−1, wherein L is the number of availablesubcarriers, whereby said modulation means (4) modulates every M-thsignal onto said subcarriers on the basis of said integer values. 10.Transmission-apparatus according to claim 6, wherein: said modulationmeans includes a switch having a first input and a second input, whereinthe first input receives a signal to be modulated onto a subcarrier andthe second input receives a zero value signal.
 11. Receiving method forreceiving OFDM-signals comprising M identical or respectively mirroredwave forms within one OFDM-timeburst, wherein M is an integer and M≧2,comprising the steps of receiving said OFDM-signals, correlating saidwaveforms to obtain time synchronization using M−1 correlators,transforming said signals into the frequency domain, and demodulatingsaid signals.
 12. Receiving method according to claim 11, characterizedin, that in said correlation step said wave form parts are correlated onthe basis of a delay value L1=S/M samples and averaged over L2≦S/Msamples, whereby S is the total number of samples in one OFDM-timeburst.13. Receiving method according to claim 11, characterized in, that aftersaid correlation step a peak detection step is carried out to providetime synchronization for said transformation of said signals into thefrequency domain.
 14. Receiving method according to claim 11,characterized in, that after said correlation step a frequency offsetdetection step is carried out to provide frequency synchronization forsaid transformation of said signals into the frequency domain. 15.Receiving apparatus for receiving OFDM-signals comprising M identical orrespectively mirrored wave forms within one OFDM-timeburst, wherein M isan integer and M≧2, comprising: receiving means for receiving saidOFDM-signals, correlating means (28, 29, 30, 31) correlating saidwaveforms to obtain time synchronization, wherein said correlation meansincludes M−1 correlators, synchronization, transformation means fortransforming said signals into the frequency domain, and demodulatingsaid signals.
 16. Receiving apparatus according to claim 15,characterized in, that in said correlation means (28, 29, 30, 31) saididentical wave forms are correlated on the basis of a delay value L1=S/Mand averaged over L2≧S/M samples, whereby S is the total number ofsamples in one OFDM-timeburst.
 17. Receiving apparatus according toclaim 15, characterized in, that after said correlation means (28, 29,30, 31) a peak detection means (46) is provided for providing timesynchronization for said transformation of said signals into thefrequency domain.
 18. Receiving apparatus according to claim 15,characterized in, that after said correlation means (28, 29, 30, 31) afrequency offset detection means (47) is provided for providingfrequency synchronization for said transformation of said signals intothe frequency domain.
 19. Transmission system for transmittingOFDM-signals, comprising: a transmission apparatus including modulationmeans for modulating said signals onto a plurality of subcarriers byOFDM-modulation, transformation means for transforming said modulatedsignals into the time domain, and transmission means for transmittingsaid signals characterized in that in said modulation means every M-thsubcarrier is modulated, wherein M is an integer greater than or equalto 2; and a receiving apparatus for receiving said OFDM-signals having Midentical or respectively mirrored waveforms within one OFDM-timeburst,including receiving means for receiving said OFDM-signals, correlationmeans for correlating said waveforms to obtain time synchronization,transformation means for transforming said signals into the frequencydomain, and demodulation means for demodulating said transformedsignals.
 20. A method for generating OFDM synchronization signals to betransmitted to synchronize a receiver device in an OFDM wirelesscommunication system, comprising the steps of: obtaining a frequencydomain sequence by modulating synchronization signals onto every 4-thsubcarrier of a plurality of available subcarriers being used in theOFDM transmission system; generating the time domain sequence byperforming Inverse Discrete Fourier Transformation onto said frequencydomain sequence into scheme to generate a time domain sequence, whereinsaid time domain sequence contains 4 identical waveforms and generatingsaid OFDM synchronization signals to be transmitted from said a timedomain sequence.
 21. A method for generating OFDM synchronizationsignals to be transmitted to synchronize a receiver device in an OFDMwireless communication system, comprising the steps of: obtaining afrequency domain sequence by modulating synchronization signals ontoevery M-th subcarrier of a plurality of subcarriers being used in saidOFDM transmission system, wherein M is an integer and M≧2, and whereinsaid frequency domain sequence comprises modulated subcarriers havingnonzero values and un-modulated subcarriers having a zero value;generating a frequency domain representation comprising said frequencydomain sequence and second frequency domain sequences having zeroamplitude symbols, and transforming said frequency domain representationinto a time domain representation by using Inverse Discrete FourierTransformation to generate said OFDM synchronization signals, whereinsaid second frequency domain sequences are arranged at the predefinedfrequency order in said frequency domain representation so that thenumber of samples in said time domain representation is equal to thenumber of symbols in said frequency domain representation.
 22. A methodfor generating OFDM synchronization signals to be transmitted tosynchronize a receiver device in an OFDM wireless communication system,comprising the steps of: obtaining a frequency domain sequence bymodulating synchronization signals onto every M-th subcarrier of aplurality of subcarriers being used in said OFDM wireless communicationsystem, wherein M is an integer and M≧2, and wherein said frequencydomain sequence comprises modulated subcarriers having nonzero valuesand un-modulated subcarriers having a zero value; generating a frequencydomain representation comprising said frequency domain sequence and asecond frequency sequence having zero amplitude symbols, andtransforming said frequency domain representation into a time domainrepresentation by a transforming unit performing a process of InverseDiscrete Fourier Transformation to generate said OFDM synchronizationsignals, wherein said modulated and unmodulated subcarriers of saidfrequency domain sequence and zero amplitude symbols of said secondfrequency domain sequence are assigned to predetermined inputs of saidtransforming unit so that so that the number of samples in said timedomain representation is equal to the number of symbols in saidfrequency domain representation.
 23. A method for generating OFDMsynchronization signals to be transmitted to synchronize a receiverdevice in an OFDM wireless communication system, comprising the stepsof: generating a frequency domain sequence by modulating synchronizationsignals onto every M-th subcarrier of a plurality of subcarriers beingused in said OFDM transmission system, wherein M is an integer and M≧2;transforming said frequency domain sequence into a time domain sequenceby using a method of Inverse Discrete Fourier Transformation, whereinsaid time domain sequence contains M-th identical waveforms; andcyclically extending said time domain sequence in time domain togenerate an extended time domain sequence; and transmitting saidextended time domain sequence.
 24. A method for generating OFDMsynchronization signals to be transmitted to synchronize a receiverdevice in an OFDM wireless communication system, comprising the stepsof: generating a frequency domain sequence by modulating synchronizationsignals onto every M-th subcarrier of a plurality of subcarriers beingused in said OFDM transmission system, wherein M is an integer and M≧2;transforming said frequency domain sequence into a time domain sequenceby using a method of Inverse Discrete Fourier Transformation, whereinsaid time domain sequence contains M-th identical waveforms so that saidtime domain sequence comprises IN-phase M-th identical waveforms andQUAD-phase M-th identical waveforms, wherein each of said QUAD-phasewaveforms is different from the corresponding each of said IN-phasewaveforms in the time domain; and transmitting said time domainsequence.
 25. A method for generating OFDM synchronization signals to betransmitted to synchronize a receiver device in an OFDM wirelesscommunication system, comprising the steps of: generating a frequencydomain sequence by modulating synchronization signals onto every M-thsubcarrier of a plurality of subcarriers being used in said OFDMtransmission system, wherein M is an integer and M≧2; transforming saidfrequency domain sequence into a time domain sequence by using a methodof Inverse Discrete Fourier Transformation, wherein said time domainsequence contains M-th identical waveforms to be correlated in thereceiver device to detect the synchronization timing informationtherefrom.